Determining top dead center
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There are a multitude of different dampers and timing pointer locations on a small block Chevy. Refer to [[Timing Tabs and Damper TDC Lines SBC]] for more on them. Other engines can refer to a service manual. | There are a multitude of different dampers and timing pointer locations on a small block Chevy. Refer to [[Timing Tabs and Damper TDC Lines SBC]] for more on them. Other engines can refer to a service manual. | ||
| − | The whole reason for doing this operation in the first place is to be able to time the engine with a timing light and know '''''absolutely''''' that the timing is correct. The elastomeric material that connects the outer inertia ring of the harmonic damper/balancer to the inner hub of the damper/balancer which presses onto the snout of the crankshaft begins to break down over time due to ozone in the atmosphere and oil and fuel or other foreign materials which may find their way onto the material. | + | The whole reason for doing this operation in the first place is to be able to time the engine with a timing light and know '''''absolutely''''' that the timing is correct. The elastomeric material that connects the outer inertia ring of the harmonic damper/balancer to the inner hub of the damper/balancer which presses onto the snout of the crankshaft begins to break down over time due to ozone in the atmosphere and oil and fuel or other foreign materials which may find their way onto the material. When this happens, the outer ring may slip circumferentially in relation to the inner hub, rendering any attempt to time the engine with a timing light futile. Even though this operation you are about to do will bring the timing marks back to correct for the time being, there is no guarantee that the ring will not slip further after a while. You also have no idea if the timing pointer matched the inertia ring in the first place if the engine has been disassembled and reassembled by someone else in its lifetime. If you want to bulletproof the operation, then start with a new or rebuilt damper and use the correct timing pointer for that damper. |
| − | + | ===Checking the outer damper ring for movement=== | |
| + | [[File:Damper line.jpg|left|400px]]Draw a sharpie line as shown in the image below. When the timing light is pointed at the TDC line, the sharpie line will also be seen. By revving the engine and running it at different speeds, if the outer ring is loose, the line on the outer ring will be seen to move independently of the line on the inner hub. | ||
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| − | + | ==Rebuilt dampers== | |
| + | One noted place to buy a rebuilt damper/balancer is [http://www.damperdoctor.com/ Damper Doctor]. They disassemble stock, OEM production dampers, clock the hub to the inertia ring and reassemble the unit with new elastomeric material pressed together under tremendous hydraulic pressure. An 8" damper for a 350 Chevy can be had for a mere $32.95 (ca. 2012). | ||
| − | The option is a used damper/balancer that may be clocked worse than the one you have or an aftermarket damper/balancer that will cost more money and may not have been correctly machined on the inner hub diameter. Some of these | + | The option is a used damper/balancer that may be clocked worse than the one you have or an aftermarket damper/balancer that will cost more money and may not have been correctly machined on the inner hub diameter. Some of these offshore (Chinese) dampers being sold are bored either oversize or undersize for the production crank snout diameter. The damper/balancer hub MUST BE A SNUG PRESS-FIT on the crank in order to properly transfer harmonics from the crankshaft to the damper/balancer hub and on to the inertia ring, where harmonics are dissipated. |
| − | On a street engine or a drag race engine down to 11.00 E.T. in the quarter mile, an OEM-type damper/balancer may be used legally. At 10.99 E.T., an aftermarket SFI-18.1 damper/balancer is required. | + | On a street engine or a drag race engine down to 11.00 E.T. in the quarter mile, an OEM-type damper/balancer may be used legally. At 10.99 E.T. and quicker, an aftermarket SFI-18.1 damper/balancer is required. Blower motors normally do not use a balancer/damper, but instead, use an aluminum toothed hub on the crank snout to drive the blower. On these blown motors, the large Gilmer drive belt functions as a dampener to dissipate crankshaft harmonics. |
===Damper suppliers=== | ===Damper suppliers=== | ||
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If the engine is a short block on the stand, you can determine TDC with a simple homemade piston stop made from a strap of metal bolted across two head bolt holes, with the strap drilled and tapped for an adjustable bolt/nut assembly. A dial indicator can also be used on a fixture that bridges the bore or on a magnetic base. This would be an ideal time to note the piston-to-deck clearance for use in computing the static compression ratio and quench distance. | If the engine is a short block on the stand, you can determine TDC with a simple homemade piston stop made from a strap of metal bolted across two head bolt holes, with the strap drilled and tapped for an adjustable bolt/nut assembly. A dial indicator can also be used on a fixture that bridges the bore or on a magnetic base. This would be an ideal time to note the piston-to-deck clearance for use in computing the static compression ratio and quench distance. | ||
| − | If the engine | + | If the engine has the heads on, use a spark plug-type piston stop tool. If there is a timing tab present, use it to mark the position with. If no tab, use a length of stiff wire that's attached to the engine to use to show the positions. This may be made easier by using a degree wheel or a timing tape on the outer ring of the damper. |
| + | #Set the plug hole-mounted piston stop to contact the piston close to TDC | ||
| + | #Rotate engine until the stop just contacts the piston- mark the location | ||
| + | #Then rotate in the opposite direction until the piston is stopped | ||
| + | #Half way between the two marks is approximately TDC | ||
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[[File:TDC1.jpg |thumb|left|Strap-type and spark plug-type piston stop tools]] | [[File:TDC1.jpg |thumb|left|Strap-type and spark plug-type piston stop tools]] | ||
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If using a camshaft with more duration, you may want to increase the ignition timing lead at the crank and limit the centrifugal advance in the distributor to achieve your total ignition timing. Some distributors, like those sold by MSD, use different bushings to control the ''amount'' of mechanical advance. The '''rate''' of mechanical advance is tailored by changing the springs and/or centrifugal weights. Usually just spring changes are all that is needed. If working on a GM HEI, the original weights are almost always a better choice than the weights sold in the various advance curve kits available from Mr. Gasket, Crane, Summit, Moroso, etc. | If using a camshaft with more duration, you may want to increase the ignition timing lead at the crank and limit the centrifugal advance in the distributor to achieve your total ignition timing. Some distributors, like those sold by MSD, use different bushings to control the ''amount'' of mechanical advance. The '''rate''' of mechanical advance is tailored by changing the springs and/or centrifugal weights. Usually just spring changes are all that is needed. If working on a GM HEI, the original weights are almost always a better choice than the weights sold in the various advance curve kits available from Mr. Gasket, Crane, Summit, Moroso, etc. | ||
| − | If you are using a radical cam and/or a converter that allows the engine RPM to come up past where you would normally limit centrifugal advance (about 2800 rpm), you may want to modify the distributor so that centrifugal advance is locked out and put your total amount of ignition advance in at the crank. Of course, the engine will not want to crank against this much ignition lead, so you will want to install a momentary switch in the wire going to the "+" terminal of the coil to disable the coil while you crank the engine. Once the engine is spinning, release the switch and the engine will fire normally. | + | If you are using a radical cam and/or a converter that allows the engine RPM to come up past where you would normally limit centrifugal advance (about 2800 rpm), you may want to modify the distributor so that centrifugal advance is locked out and put your total amount of ignition advance in at the crank. Of course, the engine will not want to crank against this much ignition lead, so you will want to install a momentary switch in the wire going to the "+" terminal of the coil to disable the coil while you crank the engine. Once the engine is spinning, release the switch and the engine will fire normally. |
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